When crude oil from a subterranean reservoir is raised to the surface and thereby reduced in pressure, solution gas is released. The quantity of gas released is dependant upon the crude oil's gas-oil ratio or GOR. Produced oil is ultimately stored in atmospheric tankage, and any associated gas which has come out of solution is typically vented from the tank. Regulatory boards are cautious regarding the quantities of gas vented from oil well sites.
For oil fields in Alberta, Canada, the Energy Resources Conservation Board (ERCB) requires an operator to continuously measure the volume of gas produced from the crude oil-producing well. An operator of a well producing only a low rate of gas may apply for an exemption from continuous measurement under ss. 14.040 and 15.140 of the Alberta Oil and Gas Conservation Regulations. This exemption is typical in heavy oil operations but also frequently occurs in conventional oil production areas. Unfortunately, at low gas rates, it is difficult to obtain gas measurement using conventional orifice-based measurement devices. One approach is to install a separator and measure the rates. Separators involve a further capital expense and require maintenance.
The objective is to measure these low gas flow rates on wells not normally equipped with separators.
More particularly, an oil well comprises a large bore casing string extending downwardly to access the subterranean oil reservoir. A production tubing string extends down the bore of the casing, forming an annulus therebetween. A downhole pump at the lower end of the tubing pumps oil up the bore of the tubing for production at the surface.
The annular space accumulates gas which is produced to lower the static pressure in the well. The gas in the annulus results from the reduction in crude oil pressure from the reservoir pressure to the annular pressure. Production of gas from the annulus is necessary to remove the produced gas which otherwise must pass through the crude oil pump and tubing string, reducing its efficiency.
Oil produced from the tubing string is reduced from the annular pressure at the pump (flowing bottom hole pressure) to the low pressure at the surface. This reduction in pressure is further associated with the release of more solution gas. The oil and released solution gas is produced from the tubing string and combined with the annulus gas flow, all of which is directed to tankage.
Therefore, in order to measure the total produced gas rate, it is necessary to measure both the annular gas and the tubing gas rates.
In the first instance, it is relatively straightforward to connect a critical flow prover or positive displacement meter to the annulus and measure its substantially liquid-free gas flow on a continuous basis prior to its joining the tubing flow. However, the tubing gas flow is not so easily measured.
The tubing gas flows concurrently with oil production and is not readily measured as a mixed liquid and gas.
Ideally, an oil-gas separator is installed for providing measurable, separate gas and oil flow rates. However, many sites do not incorporate a separator due in part to low produced flow rates, the cost or the requirement for ongoing maintenance. Accordingly, the gas rate may not be directly measured.
For conventional oil production, the ERCB requires a representative 24-hour production test in order to establish eligibility for exemption and determination of an appropriate GOR to be used for ongoing production purposes. The 24-hour test typically comprises temporarily installing a temporary oil-gas separator in-line and determining the relative flows of oil and gas. Should an exemption be granted, annual 24-hour tests are required to determine continuing eligibility and to update the GOR value.
For the annual tests the ERCB states that consideration should be given to using positive displacement meters for conducting GOR tests at gas rates below 500 m.sup.3 /d. In accordance with the invention, a graphical method of determining the gas rate is provided which eliminates the need for supplementary equipment, and significantly reduces time required for testing as prescribed by the ERCB. As an added benefit, gas-oil ratio information for the crude oil is determined which is of significant reservoir engineering importance as diagnostic tool for monitoring and implementing reservoir depletion strategies.